The invention relates to a converter and a method for controlling a converter.
High-speed synchronous machines require feed frequencies of 1000 Hz or higher, even in two-pole design. Feeding units used in this case include for example voltage-impressing, two- or three-phase pulse-controlled inverters, which can operate according to the subharmonic method, for example.
In the case of space vector modulation or else in the case of types of modulation with a triangular carrier signal (subharmonic method), the voltages provided contain, alongside the desired fundamental, harmonics having the frequenciesfa=μfswitch±ηfGS.
In this case, fGS is the frequency of the fundamental, fswitch is the pulse frequency, and μ, η are integral, positive ordinal numbers. The components in the output voltage having the frequencies fa which arise in addition to the fundamental on account of the modulation are referred to as modulation products.
As a result of the higher frequency components of the voltages in the range of the pulse frequency and above, higher-frequency components likewise occur in the conductor currents. These distortion currents brought about by higher frequency components generate additional Joule heat and core losses in the machine.
The literature, for example EP 1035642 A1, describes numerous methods which make it possible, in the case of variable-speed drives, to reduce the harmonic currents or to avoid low-frequency components. In the modulation methods, by way of example, individual switches in the inverter are not switched for specific angle ranges of the fundamental (flat-top modulation) and the average switching losses that arise are lower than if each switch is operated at pulse frequency.
A different method works with optimized pulse patterns. In this case, the harmonic behavior is influenced directly, e.g. by eliminating specific voltage harmonics or minimizing the square harmonic current route-mean-square value. This is done by iteratively determining from the Fourier coefficients of the inverter output voltage, taking account of the pulse number and the modulation factor, switching angles for a pulse pattern with quarter-cycle symmetry which lead to the elimination of the desired harmonics.
A further possibility for keeping the distortion currents small is forwarded by the three-point inverter for the same intermediate circuit voltage UZ. With said inverter, in contrast to the two-point inverter, a three-stage modulation can be carried out, with the result that during a half-cycle of the fundamental frequency, the output voltage can assume not just two (UZ, 0; −UZ, 0) but three different voltage values (UZ, Uz/2, 0; −UZ, −UZ/2, 0).
In the case of high-speed drives having a constant rotational speed, i.e. constant fundamental frequency and constant pulse frequency, filters, e.g. series resonant circuits, are used for reducing the modulation products.
DE 103 23 218 A1 discloses a high-voltage converter whose output is connected to a medium-frequency transformer. A converter connected downstream of the medium-frequency transformer comprises, in DE 103 23 218 A1, an input power converter, a DC voltage intermediate circuit and a pulse-controlled inverter. Likewise, DE 103 23 218 A1 explains a method for driving the high-voltage converter.
Pulse-controlled converters with a filter are used in many applications. This concerns, in particular, arrangements with feedback capability for feeding a DC voltage intermediate circuit for instance from a three-phase power supply or else arrangements for feeding a rotating-field machine such as e.g. an asynchronous motor or else a synchronous motor with separate excitation or permanent-field excitation. The sinusoidal filters are used for example in order that the winding loading of the motor or else the EMC influencing (EMC-electromagnetic compatibility) is kept small.
Filters can have a pronounced resonance at a resonant frequency. Frequencies below the resonant frequency can pass through the filter. Above the resonant frequency, the voltage components are attenuated more or less depending on distance from the resonant frequency.
A strong magnification is effected in the range of resonance. The magnification is dependent on the attenuation of the filter. In general, the filters are only weakly attenuated since the losses in the filter rise with the attenuation. The weaker the attenuation, however, the greater the resonance magnification. Therefore, it is necessary to minimize any excitation in the range of the resonance magnification. The filter is designed by the choice of the resonant frequency of the filter in such a way that the fundamental lies in the passband (that is to say below the resonant frequency) of the filter. Modulation products in the converter voltage lie exclusively above the resonant frequency and are thus filtered out. The resonant frequency therefore lies above the fundamental and below the modulation products excited by the modulation. Correspondingly large coils and capacitors are used for said resonant frequency.